The NS3 serine protease of HCV is a multifunctional protein which contains a seine protease domain and a RNA helicase domain. The protease cofactor NS4A, which is a relatively small protein, is absolutely required for enhanced serine protease activity. The NS3 serine protease is essential in the viral lifecycle. From analysis of the substrate binding site as revealed by X-ray crystal structure, it has been shown that the binding site of the NS3 protease is remarkably shallow and solvent exposed making small molecule inhibitor design a challenge.
It is believed that two HCV protease inhibitors have entered clinical trials, namely Boehringer Ingelhelm's BILN-2061 disclosed in WO 0059929 and Vertex′ VX-950 disclosed in WO 0387092. A number of similar peptidomimetic HCV protease inhibitors have also been proposed in the academic and patent literature. Common for the vast majority of such prior art peptidomimetics is the presence of an L-proline derivative at the P2 position of the inhibitor and interacting with the S2 subsite of the HCV protease enzyme. In the case of BILN-2061, the L-proline is 4-substituted with a quinoline ether, whereas VX-950 has a carboyclic ring fused to the L-proline ring. Most peptidomimetics additionally comprise additional L-amino acid derivatives peptide bonded at the P3 position, with many proposed inhibitors also including additional L-amino acid derivatives extending into P4, P5 and P6.
It has already become apparent that the sustained administration of BILN-2061 or VX-950 selects HCV mutants which are resistant to the respective drug, so called drug escape mutants. These drug escape mutants have characteristic mutations in the HCV protease genome, notably D168V, D168Y and/or A165S. Treatment paradigms for HCV will thus have to resemble HIV treatment, where drug escape mutations also arise readily. Accordingly, additional drugs with different resistance patterns will consistently be required to provide falling patients with treatment options, and combination therapy with multiple drugs is likely to be the norm in the future, even for first line treatment.
Experience with HIV drugs, and HIV protease inhibitors in particular, has further emphasized that sub-optimal pharmacokinetics and complex dosage regimes quickly result in inadvertent compliance failures. This in turn means that the 24 hour trough concentration (minimum plasma concentration) for the respective drugs in an HIV regime frequently falls below the IC90 or ED90 threshold for large parts of the day. It is considered that a 24 hour trough level of at least the IC50, and more realistically, the IC90 or ED90 is essential to slow down the development of drug escape mutants and achieving the necessary pharmacokinetics and drug metabolism to allow such trough levels provides a stringent challenge to drug design. The strongly peptidomimetic nature of prior art HCV protease inhibitors, with multiple peptide bonds in native configurations poses pharmacokinetic hurdles to effective dosage regimes.